This invention relates generally to the utilization of Navstar global positioning system (GPS) signals to determine position. Specifically, the invention relates to methods and devices for delivering and receiving differential GPS correction for more accurate determination of position.
The Navstar GPS is a navigation system for ascertaining position based upon signals received from orbiting satellites. These satellites, financed by the US Department of Defense, transmit accurately timed gigahertz signals in the L-band from which the user can determine position through timing and triangulation techniques. Although the Navstar system is considerably more accurate than most other radio navigation systems, it is subject to undesirable limitations for a variety of reasons. First, physical phenomenon introduce errors. Since the satellite signals are transmitted through the earth's atmosphere, they are subject to distortions, delays, relativistic effects, and other physical impacts which degrade accuracy. In addition, equipment limitations can impact accuracy. For instance, since many GPS receivers use inexpensive internal clocks, timing may not be as accurate as desired.
Perhaps the most significant limitation in accuracy is due to the fact that limitations are purposefully built into the system for national security. Since the Navstar satellites are primarily for military purposes, the US Department of Defense has purposely limited the accuracy of these signals for civilian users. This is facilitated by providing two different signals: a precision code signal (P-Code) available for government use and a course acquisition code signal (C/A-Code) for civilian use. The accuracy of the P-Code signal at a 95% probability level without further correction is on the order of 25 to 30 meters. The Department of Defense selectively introduces an error in to the C/A-Code signal so that a similar accuracy is on the order of 100 meters (330 feet). Naturally, since civilian users do not have access to P-Code signals, this accuracy is of significant concern as commercial applications expand.
To improve upon the accuracy of the Navstar system, one technique has been developed by groups such as the United States Coast Guard--differential GPS navigation. This technique is based upon the fact that both inherent and intentional errors are approximately the same within a given locale. By receiving the GPS signals at a known location, the error in location can be determined and transmitted to users in the vicinity of that known location. This error, potentially represented by a set of correction values such as satellite ephemeris and clock bias errors for each satellite, can greatly improve the accuracies of the system. For instance, utilizing differential GPS techniques in conjunction with other corrections, accuracies of considerably less than one meter have been achieved. To transmit the differential GPS correction, a variety of techniques have been suggested including satellite, person-to-person, and ground based transmissions. Typically, the ground based techniques have involved dedicated transmitters using specific frequency allocations and licensing. Due to limited frequency availability, this has resulted in limiting the useful transmission range of such systems.
In attempting to provide a more accurate determination of location, prior efforts have naturally focused on providing as accurate a correction as possible. Unfortunately the technical focus of this approach has in many instances caused those involved to overlook the commercial realities necessary to effectively implement a differential GPS system on a widespread basis. While those involved have made great technical strides and even developed correction techniques capable of centimeter level accuracies, they have not developed a system which can be practically implemented. Problems such as the initial expense of such systems, the recovery of the cost of placing and maintaining such systems, and the equipment impacts for users who may not require maximum accuracy have not been fully addressed. As a result, differential GPS corrections have not yet become available for widespread use.
Certainly, those involved in the industry have appreciated the desirability of providing differential GPS correction in a cost effective manner. To date however, they have not fully satisfied this desire. This may be due, in part, to a preoccupation with the technical challenges, limitations in range and reliability, or to presumptions regarding attaining the maximum accuracy in a single differential GPS signal. As a result those involved in providing differential GPS signals, have overlooked the solutions proposed by the present invention even though the needed implementing arts and elements had long been available.
Not only have the technical directions taken by those involved with differential GPS navigation lead away from the direction taken by the present invention, but the directions used in unrelated arts have also done so to some extent. For instance, as explained later, the invention may involve the use of subcarrier broadcasts to provide differential GPS correction. Within this art, pre-existing standards for the broadcast of data have been established. These standards might suggest that an adequate transmission rate is not available for differential GPS correction. The present invention establishes that, in fact, such potential limitations do not exist.
Basically, it appears that to some degree attempts at providing differential GPS correction in a commercially feasible manner have, until the present invention, been limited because those skilled in the differential art have simply failed to understand that the problem of delivery was not purely technical in nature. To an extent they failed to address the need for cost recovery and failed to realize that multiple accuracy corrections could afford significant commercial advantages. Each of these contribute to a practical system which can be implemented on a widespread basis. Thus, until the present invention, those skilled in the art simply failed to realize that simple modifications of existing concepts could achieve a solution. Until the present invention, no system achieved a practically balanced delivery of differential GPS correction let alone multiple differential GPS corrections with the commercially attractive features of the present invention.